Sensors are used to provide positional information for use by servomechanisms. Servomechanisms are used in a plurality of control systems that utilize feedback control. Such sensors comprise various technologies, including: optical, inductive, mechanical, acoustic, or capacitive-type technologies. However many of these sensor technologies cannot survive in the harsh environments typically exposed to work vehicles, e.g. extreme temperature, humidity, dust, oil, moisture, vibration, shock, etc.
One example of a servomechanisms for a work vehicle is an electrohydraulic valve system. Electrohydraulic valve systems provide the muscle for high-force applications. An electromechanical actuator provides the necessary linear or rotary motion to displace the spool of an hydraulic valve to a desired position. Typically a position sensor measures the position of the actuator armature to achieve feedback control.
The most common method of determining the position of the armature is to connect an external sensor to the actuator. Such sensors often take the form of linear voltage differential transformers (LVDT). While the addition of the LVDT provides the desired information, the excessive cost of the sensor due to the associated complex electronic circuitry and EMI shielding requirements make the LVDT undesirable.
Due to the inherent simplicity of capacitive technology, it may be desirable to use such technology in work vehicle applications. Capacitive technology includes the advantages of a non-contacting sensor design that lends to long term reliability. Additionally, capacitive technology can be used in an hydraulic environment to which the entire sensor can be fully immersed in hydraulic fluid and still provide good accuracy.
However existing capacitive sensor technology has several drawbacks. For example, such capacitive sensors utilize moving electrically conductive elements to give positional information. Unfortunately the moving elements tend to cause poor sensor reliability because the movement weakens the electrical connections. Further the moving elements also introduce unwanted "noise" to the control system. Other sources of problems with existing capacitive sensor technology include the addition of circuitry to deliver an "input" signal to the capacitive elements, and circuitry to condition the "output" signal. The additional circuitry adds excessive cost and complexity to the sensor.
The present invention is directed to overcoming one or more of the problems as set forth above.